docs: add testing philosophy to coding guideline

and update groups architecture docs #371

CODE_GUIDELINES.md

@@ -1515,6 +1515,40 @@ mix test test/membership/member_test.exs:42
 
 ### 4.7 Testing Best Practices
 
+**Testing Philosophy: Focus on Business Logic, Not Framework Functionality**
+
+We test our business logic and domain-specific behavior, not core framework features. Framework features (Ash validations, Ecto relationships, etc.) are already tested by their respective libraries.
+
+**What We Test:**
+- Business rules and validations specific to our domain
+- Custom business logic (slug generation, calculations, etc.)
+- Integration between our resources
+- Database-level constraints (unique constraints, foreign keys, CASCADE)
+- Query performance (N+1 prevention)
+
+**What We Don't Test:**
+- Framework core functionality (Ash validations work, Ecto relationships work, etc.)
+- Standard CRUD operations without custom logic
+- Framework-provided features that are already tested upstream
+- Detailed slug generation edge cases (Umlauts, truncation, etc.) if covered by reusable change tests
+
+**Example:**
+```elixir
+# ✅ GOOD - Tests our business rule
+test "slug is immutable (doesn't change when name is updated)" do
+  {:ok, group} = Membership.create_group(%{name: "Original"}, actor: actor)
+  original_slug = group.slug
+  
+  {:ok, updated} = Membership.update_group(group, %{name: "New"}, actor: actor)
+  assert updated.slug == original_slug  # Business rule: slug doesn't change
+end
+
+# ❌ AVOID - Tests framework functionality
+test "Ash.Changeset validates required fields" do
+  # This is already tested by Ash framework
+end
+```
+
 **Descriptive Test Names:**
 
 ```elixir